Chapter 8 - Natural Ventilation in Cold Weather

Controlling naturally ventilated buildings during periods of cold weather can be challenging. Excessive air infiltration will substantially increase the need for supplemental heat.

For a 10 mph outside wind speed acting directly on the side of a building, an equivalent crack ¾ inch along the entire length of both sides of the building would be sufficient to supply the required amount of fresh air through the building. However, maintaining a gap this narrow down hundreds of feet of building length is not easy due to normal construction irregularities and flexibility of curtain system rods. For example, if one end of a 200-foot curtain is cracked ¼ inch while the other end is cracked open about 1 inch, the average opening along the length of the building is ¾ inch. But the wider cracker will provide more cold air to that section of the house, and the narrower crack will result in an under ventilated section of the house. A modified top panel on the curtain system or a separate baffle inlet system can be used to provide intermittent openings along the house length to allow better control of low airflow direction.

Minimizing cold-weather infiltration

The greatest amount of heat loss in a curtain-sided poultry house will be through the sidewall curtains. This loss is due primarily to the low insulating value of the curtains, yet the situation can be made even worse when the curtain does not seal tightly against the sidewall. In houses with very loose curtain straps or ropes, it is not uncommon to see an inch or more of curtain movement away from the house. Even a ¼ inch crack running the length of a 500-ft house provides the equivalent area of a 10 ft² opening in the sidewall.

Curtain straps or anti-billow ropes should be fastened an inch or two above the curtain rod rests at the top of the sidewall when the curtain is completely closed. Fastening straps six or more inches above the top of the curtains will not hold it tightly in place against the house. The bottom of the curtain must also be tight. For a double-hemmed curtain, fasten the strap just below where the bottom rod rests when the curtain is closed. For single-hemmed curtain, nail a barren board along the bottom at the base of the curtain opening; then nail the bottom of the curtain strap far enough below the sidewall opening so that when the curtain is lowered, the strap will not obstruct the opening. The life of straps can be increased by reducing their flapping in the wind. To do this, nail the bottom of the strap, give it a half twist and then nail the top of the strap in place. Check curtain straps for tightness each fall, since the curtain shrinks and straps loosen over time.

Envelopes at the curtain ends should extend at least 12 inches in from each curtain end and be placed as tightly as possible against the curtain while still maintaining an adequate pocket for a fully open curtain. Envelopes can be made from leftover curtain material or treated plywood. Both methods work very well. Windshields can be installed at the top of the sidewall. Windshields should be made of treated plywood and securely fastened to the sidewall top. Extending the shield six inches below the top of the opening will provide an excellent barrier.

One of the simplest and most effective infiltration control techniques is overlapping the curtain at the top and bottom of the sidewall openings. Providing a 6-inch overlap at the top and bottom of the sidewall opening will reduce infiltration considerably. This will also accommodate the natural tendency of the curtain material to shrink while providing some overlap. One or two inches of overlap are not enough to reduce infiltration. Over a few years, curtains will shrink a couple of inches on an 8-ft curtain. Providing overlap requires purchasing a curtain that is roughly one foot wider than the maximum sidewall opening size. The added expense will be more than made up for by the savings in supplemental heat required to warm infiltration air. Make sure limit switches on the curtain machines are set to maximize curtain overlap.

On a cold, windy day it is possible that more heat is lost through loose-fitting curtains than through the timer fans set for minimum ventilation. The wind may be creating an opening at the top of the curtain by pulling the curtain and rod away from the house by suction forces. A ½ inch opening the entire sidewall length from wind suction is not uncommon. This problem is frequently the result of leaving curtain straps loose to ensure that sidewall curtains open and close easily. In windy conditions, the windward curtain will be pushed against the wall while the downwind curtain pulls away from the sidewall. It curtain straps or strings are loose the curtain can pull an inch or more off the side of the house and form a large, uncontrolled air exhaust opening. Although some air leakage can be minimized by the overlap and windshield strategies described above, infiltration due to loose-fitting curtains can occur when the curtain is overlapped.

Some air leakage can be minimized by the overlap and windshield described above. Another method is to install a curtain pocket for the top of the curtain to slide into when closed. A hemmed curtain 12-18 inches long with a rod is attached a few inches above where the top of the curtain rod rests when closed. This short curtain pocket is installed over the curtain straps or strings, so that it does don’t interfere with the curtain movement. If sidewall curtains are not raised and lowered often (as in a hybrid mechanical and natural ventilation situation), the strings or straps may be installed over the pocket for a slightly tighter seal.

Multi-layered curtains provide better infiltration control than single layer curtains. Constructing envelopes at the curtain ends and a windshield or curtain pocket near the upper opening can dramatically reduce infiltration. Adjust curtain straps so that the curtain closes evenly and maintains a sufficient overlap along its entire length. This will also help sidewall curtains open uniformly to provide inlets that will keep house temperature and fresh air exchange more uniform along the curtain length. Consider sealing any side and endwall doors that will not be used with tight-fitting plastic to reduce infiltration. Gaps in the structure that are large enough to be seen through are obviously large enough to allow significant air infiltrations. Gaps around doors, fan housings, and so on should be sealed with foam-in-place insulation.

Sidewall curtains used in naturally ventilated buildings provide a downward direction to the air stream entering through a small curtain opening. This can be a significant disadvantage during cold weather. Fresh air is not warmed before it reaches the floor. This can set off heater thermostats as a wave of cold air passes through the brooding zone. One solution is to install a separate baffle inlet system above the curtain sidewall inlets. These inlets can be used during the coldest periods to direct incoming air toward the ceiling. Hybrid systems combining natural and mechanical ventilation use this technique more often than entirely naturally ventilated facilities.

The distance that fresh air will travel in the building depends on its temperature in relation to the indoor air. Cold air will fall upon entering the building sidewall opening because it is heavier than inside warm air. Incoming air of mild temperatures will fall as well, but it will fall further across the building width. Air that is warmer or very near the inside temperature will remain nearly horizontal, or in some cases, can rise when it enters the building.